In recent years various recording methods have been proposed which utilize the color forming reaction between an electron donating basic colorless chromogenic material and an electron accepting acidic reactant material. Especially microcapsules enclosing droplets of hydrophobic oily solutions of colorless chromogenic materials were widely used for pressure sensitive manifold paper, etc.
The chromogenic material to be encapsulated must be capable of exhibiting good color forming properties when recording and giving color records with high stability. Additionally the chromogenic material must remain stable when formulated into a hydrophobic oily solution and then dispersed and emulsified in an aqueous medium for encapsulation and must be amenable to efficient encapsulation.
Diarylmethane derivatives represented by the formula ##STR3## wherein L and M are each 1,4-arylene with or without a substituent, R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each alkyl, cycloalkyl, aralkyl or aryl with or without a substituent, and R.sub.5 is alkyl, cycloalkyl, aralkyl, aryl or heteroaryl with or without a substituent are synthesized by a reaction in an aqueous medium, for example, by the reaction of a corresponding diphenylcarbinol with a sulfinic acid in an acidic aqueous solution (Beilstein Organische Chemie, Vol. 13, p. 704), or by the reaction of auramine with a sulfinic acid in an acidic aqueous solution (Japanese Kokai No. 95420/1973). With these processes which use an aqueous medium, the reactants to be reacted with sulfinic acids are limited to diphenylcarbinols or auramine, so that the processes involve the problem that the starting materials are available only from limited sources. Additionally preparation of hydrophobic organic solvent solutions of such diarylmethane derivatives requires the step of isolating the synthesized diarylmethane derivative from the aqueous medium and the subsequent step of purification. These isolating and purifying steps inevitably entail a reduction in the yield of the synthesized derivative and permit degradation of part of the derivative during the procedure. The diarylmethane derivatives of the formula (III) have another drawback that when a hydrophobic organic solvent solution of the derivative is dispersed in an aqueous medium for encapsulation, the derivative markedly dissolves out into the aqueous medium, with the result that the quantity of the derivative to be encapsulated as the chromogenic material substantially reduces.
An object of the invention is to eliminate the foregoing drawbacks involved in the synthesis and encapsulation of the diarylmethane derivatives of the formula (III) and to provide a process for synthesizing and encapsulating the derivatives with a greatly improved efficiency.
Another object of the invention is to provide a process for preparing microcapsules by which colorless chromogenic materials can be synthesized and encapsulated at the same time by a single step very efficiently.
These and other objects of the present invention will become apparent from the following description.